Zen Cart™ - Putting the dream of your own business within reach of anyone!

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This error generally means that one or more system tables cannot be found in the database.

This can occur if you've provided valid DB credentials in your configure.php files and the database itself exists but contains no tables. In this case you need to run zc_install to properly setup the database contents.

Further, this symptom most commonly occurs when moving your site to another server and not properly preserving the DB_PREFIX setting from the old site's configure.php into the new site's configure.php, thus the tablenames cannot be located because they don't match up properly. Or you need to run zc_install to create the proper tables. Or you need to import the correct matching database data, since what you've got in the database presently isn't suitable for your current DB_PREFIX configuration.
(To manually figure out the correct prefix to use, look at the database using phpMyAdmin and see what common 2-4 letter prefix starts all the tablenames. Then set that value into the DB_PREFIX definition in your configure.php file.)

Many storeowners are uncertain whether they should add SSL to their site, citing various reasons, including cost and complexity of setup.

MODERN ANSWER: Yes, you should use SSL on your site. Not only does it lend to protecting sensitive information, but it is also becoming more and more important for good search engine ranking nowadays.

Now for the technical and bigger-picture explanation:

Here are the things to consider:

WHY USE SSL?
SSL encrypts communications between your customer's browser and your webserver. This means nobody can snoop on what they're transmitting to you (such as someone spying on internet traffic in a cafe or wifi hotspot, or library)

DO I NEED SSL?
If your site is collecting credit card info directly in a page inside your store (ie: not redirecting to a bank or payment gateway site to collect card info for payment) then YES you absolutely MUST use SSL to protect your customers' payment information.
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Situation:
The following error occurs when trying to process a payment through Paypal express near the final stages.

We are sorry for the inconvenience; however, at the present time we are unable to use PayPal to process orders from the geographic region you selected as your PayPal address. Please continue using normal checkout and select from the available payment methods to complete your order.
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Zen Cart's use of cookies is VERY simple: it only sets and retrieves an anonymous session cookie. Nothing more.
However, alterations added by the storeowner may behave differently, including but not limited to tracking addons, analytics scripts, tracking pixels, etc. Those are up to the storeowner to disclose.